FTP details are hostname/username/password for accessing your files on the server by using FTP client (similar to FileZilla).
The FTP details will comprise of a server address (eg. ftp.yourdomain.com), username and password. If you don't have them or don't know what they are, you can simply contact your web hosting provider and they will give you this information.
FTP details example:
In case you don't have a hosting server, you can consider purchasing it with our partner BlueHost by this link.